开云体育

I think everyone’s on the same track here; it’s just a matter of semantics.? This is my understanding based on my own non-engineering experience, which should provide my fellow experimenters with opportunities for correction and clarification.

Given:? A transformer-coupled loop antenna operating at LW, MW or SW frequencies having a resonant primary and a non-resonant secondary.

I think there are really two separate things at work here:

???? a) the impedance match between the two windings, and

???? b) the coefficient of coupling between primary and secondary, 0 to 1.

Impedance Match:? The goal for the coupling (secondary) coil is to have an output impedance roughly equal to that of the transmission line and ultimately the receiver - say 50 ohms for example.? The primary circuit at resonance produces a reactance which is a function of the frequency and coil inductance.? As the frequency goes up, so does the inductive reactance.? Although within the circuit the inductive and capacitive reactances cancel one another, each is still present, otherwise there would be no resonance.? It is this impedance which has to be transformed by the secondary.

Thus, the secondary must have the correct reactance ratio with the primary in order for optimum signal transfer to occur.? As the tuned frequency of the resonant circuit becomes higher, this ratio must also be higher.? Since the inductance of the primary coil is constant, the inductance of the secondary (coupling) coil must be decreased in order to increase the ratio.

At LW and MW frequencies, the tuning range is not that great, and a single value of inductance will normally produce acceptable results.? At SW frequencies however, the tuning range of the antenna may be several MHz and the change in reactance of the primary coil can be significant.? In my SW loops I have incorporated tapped coupling coils, which have the highest inductance (lowest primary-secondary ratio) at low frequencies and least inductance at the highest portion of the covered spectrum.

Coefficient of Coupling:? This deals with the actual physical size and orientation of the coupling coil in relation to the primary coil, regardless of its inductance; that is, how much of the alternating magnetic field generated by the primary circuit is captured by the coupling coil?? On one hand, a small coil will not capture enough of the primary circuit’s magnetic field for adequate signal transfer to the transmission line and receiver; on the other hand, too large a coil will significantly load the primary circuit and lower the Q.

Without any available guidance, this is where experimentation comes in.? Numerous Internet sites have advocated the 1/5 rule, and this seems to be as good a place as any to start.

Bob C.